1. Field of the Invention
The present invention relates to a surface-mount-type SAW filter module in which a SAW chip of an electronic part utilizing a Surface Acoustic Wave (SAW) on a piezoelectric substrate is built in as a filter, such as an antenna switching circuit.
2. Description of the Related Art
Recently, in antenna switching circuits or the like for transmitting and receiving high frequency signals, a SAW filter for selectively passing signals of specified frequencies has been extensively employed as a transmission filter and a reception filter. The SAW filter comprises a SAW chip having a predetermined electrode pattern, electrically connected thereto, formed on a surface of a piezoelectric substrate such as quartz. However, the SAW chip is a relatively large chip component, and when a SAW filter module such as an antenna circuit unit is manufactured by arranging a large number of circuit components including the SAW chip, the size of a module is less easily reduced if the SAW chip is mounted on an outer surface of the substrate.
Thus, a SAW filter module that can be miniaturized by placing the SAW chip in a recess of the substrate has been proposed, as shown in FIG. 4. The SAW filter module shown in FIG. 4 mainly comprises a multi-layered substrate (LTCC) 1, a SAW chip 2 mounted in a recess 1a of the multi-layered substrate 1, various kinds of chip components 3, such as a chip resistor and a chip capacitor, mounted on the top surface of the multi-layered substrate 1, end-face electrodes 4 extending from side surfaces of the multi-layered substrate 1 to the bottom surface and top surface thereof, and a sealing plate 5 such as a metal plate soldered at a certain position for closing-off the recess 1a of the multi-layered substrate 1.
The multi-layered substrate 1 is obtained by laminating green sheets formed of a low-temperature co-fired ceramic material, baking them at a temperature not higher than 1,000xc2x0 C., and mounting them on a mother board (a base substrate), which is not shown. The SAW chip 2 is connected to a lead pattern 7 by a bonding wire 6, and the lead pattern 7 is led to the top surface of the multi-layered substrate 1 via a through hole (not shown). The SAW chip 2 and various kinds of chip components 3 are electrically connected to an external circuit via the end-face electrodes 4 by connecting the SAW chip 2 and various kinds of chip components 3 to the end-face electrodes 4 on the top surface of the multi-layered substrate 1. The SAW chip 2 is bonded to the top surface of a recess 1a of the multi-layered substrate 1, and the recess 1a is filled with inert gas such as nitrogen.
In recent years, the technology for miniaturizing circuit components, such as chip components and semiconductor components has advanced considerably, and even a SAW filter module of the conventional structure shown in FIG. 4 can be considerably miniaturized by mounting these ultra-compact circuit components on the substrate at high density. However, miniaturization of the circuit components, such as chip components and semiconductor components, is limited, and reduction of the pitch between components is also limited because care must be taken to ensure that the soldered parts of each circuit component are not short-circuited when a large number of circuit components are mounted on the substrate. Thus, SAW filter modules of the conventional structure have suffered from a problem in that the demand for miniaturization in recent years are less easily realized, and low profile devices are not easily realized when mounting chip components.
Accordingly, the present invention has been achieved in light of the circumstances of the conventional technology described above, and it is an object of the present invention to provide a SAW filter module in which miniaturization and low profile can be easily achieved, without adversely affecting the reliability.
In order to achieve the above object, a SAW filter module of the present invention comprises a substrate, having a recess, mounted on a mother board, a SAW chip mounted in the recess of the substrate, a sealing plate soldered to the substrate to close-off the recess, an insulation layer having a smooth surface disposed on a surface opposite to the sealing plate side of the substrate, circuit elements formed by a thin film technique on the insulation layer, and an end-face electrode which is disposed on an external surface including a side surface of the substrate which is connected to the SAW chip and the circuit elements, thus allowing the SAW chip to pass specified signals.
In the SAW filter module of the above-described configuration, circuit elements such as capacitors and inductors can be formed on the insulation layer with high accuracy by using a thin film technology after providing the insulation layer on one side of the substrate to smooth the surface. Therefore, chip components whose pitch between components is severely limited, can be reduced in number, and thus miniaturization and low profile of the entire module can be easily achieved.
In the above configuration, if a first through hole for connecting the circuit elements to the end-face electrode is formed through the substrate and the insulation layer, and a second through hole for connecting the SAW chip to the circuit elements is formed through the substrate and the insulation layer, miniaturization of the entire module is not prevented when connecting the end-face electrode, the circuit elements, and the SAW chip to each other, and a reliable connection can be easily ensured. For example, in an antenna circuit unit in which a plurality of circuit elements constituting an antenna switching circuit are formed on the insulation layer by a thin film technique, and the SAW chip connected to these circuit elements functions as a filter, miniaturization can be easily achieved and reliability can be easily improved by providing the first and second through holes.
A part for allowing these through holes to penetrate through the insulation layer may be provided before forming the insulation layer on the substrate, or it may be provided after forming the insulation layer on the substrate. This means that, if a protruding deposited part is formed on one end on the circuit element side of each through hole before forming the insulation layer, the deposited part penetrates through the insulation layer as a result by coating the insulation layer on the substrate except for the area where the deposited part is placed.
Alternatively, a part corresponding to one end on the circuit element side of each through hole may be formed by forming a hole in the insulation layer by laser trimming or the like after coating the insulation layer on the entire surface of the substrate.
In addition, in the above configuration, if all chip components including the SAW chip are mounted in the recess of the substrate, the chip components can be completely eliminated from the external surface of the multi-layered substrate, and a low profile SAW filter module can be achieved.
Circuit elements such as capacitors and inductors can be formed with high accuracy on an insulation layer using thin film technology after providing the insulation layer on one side of the substrate and smoothing the surface thereof, and a SAW filter module can be provided thereby, in which the number of chip components whose pitch between components is severely limited can be reduced. As a result, miniaturization can be easily achieved, and the reliability is not reduced.